This invention relates to a plastic material extruder and more particularly an apparatus for controlling the temperature of the inner barrel of the extruder as related to the temperature of the outer sheath in which heating and cooling means are contained.
In order to produce uniform products from an extruder, it is advantageous when processing plastic material that the barrel be kept at a constant temperature. Changes in the barrel temperature change the pastic material and particularly its viscosity which can cause uneven flow. It is also known that the barrel and the sheath which contains the heating and cooling devices, act as heat sinks; thus as the temperature of barrel and in turn the plastic resin material fluctuates, the flow varies. There is a time lag in prior devices which occurs so that the barrel may be either too hot or too cool. Basically, barrel temperatures can swing over a relatively large range, and in the prior art devices, the temperature control devices have had a very slow response to reduce temperature excursions. This is unsatisfactory with many plastic resin materials.
The problem has been previously recognized and dealt with as, for example, in U.S. Pat. No. 3,751,014 where a pair of temperature probes are used, one in the barrel and one in the sheath, and the two temperatures sensed are averaged. That is, the control is sensitive to T-1 plus T-2 divided by two. A similar approach is seen in U.S. Pat. No. 3,866,669.
It is therefore an object of the instant invention to provide an improved method and apparatus for controlling the temperature of the plastic to within a narrow band by sensing the temperature of the inner barrel and the other sheath by readjusting the set point of the outer surface from the actual outer surface temperature by a deviation of error of the inner barrel from its set point.